1.Field of the Invention
The present invention relates generally to high temperature well cement compositions and methods, and more particularly, to set retarded cement compositions which are substantially non-thinning at high temperatures and methods of using such compositions.
2. Description of the Prior Art
In the primary cementing of oil, gas and geothermal wells, casing and/or one or more liners are cemented within the well bore. The cementing is accomplished by introducing a cement composition into the annular space between the casing and/or liner and the walls of the well bore. Generally, the cement composition is pumped downwardly within the casing and/or liner to the bottom thereof and then upwardly into the annulus. Once placed in the annulus, the cement composition sets into a hard mass whereby the casing and/or liner is bonded to the earth formations forming the walls of the well bore.
Because one or more of the formations or zones penetrated by the well bore may contain fluids under pressure, the cement composition density is often increased by including a weighting material such as hematite therein to increase the hydrostatic pressure exerted by the composition on the formations. One or more set retarding additives are also usually included in the cement composition in order to allow adequate time for mixing the composition on the surface and then pumping it down the well bore and into a desired location therein. While set retarding additives successfully extend the time between mixing and the setting of cement compositions, they are also temperature sensitive. That is, the higher the temperature to which a cement composition including such an additive is subjected, the greater quantity of the additive required. In high temperature wells wherein the cement composition must contain relatively high quantities of a set retarding additive, excessive thinning of the cement composition often occurs which leads to the settling of solids such as weighting materials in the cement composition as well as the separation of free water therefrom which can bring about undesirable results such as the formation of a non-uniform density cement column, bridging, water pockets within the set cement column, poor bonding, etc.
The high temperature thinning of set retarded cement compositions has been reduced heretofore by combining one or more viscosifying agents with the cement compositions. However, the quantities of viscosifying agents which can be utilized in cement compositions are limited because the viscosifying agents impart high surface viscosities to the cement compositions which in turn makes mixing and pumping difficult. Thus, even where one or more viscosifying agents are included in a cement composition, because of the limited amounts of such agents which can be used excessive thinning at high temperatures still results.
More recently, the thinning of a set retarded cement composition a high temperatures has been combated by including a delayed viscosifying agent in the composition which yields viscosity only at the high temperatures encountered. U.S. Pat. No. 4,997,487 issued on Mar. 5, 1991 discloses such a delayed viscosifying agent comprised of a galactomannan gum which has been treated with a hydrophobing agent. While the delayed viscosifying agent functions to make set retarded cement compositions formed with fresh water non-thinning at high temperatures, the agent is relatively ineffective in saturated salt water cement compositions. Saturated salt water cement compositions are used to cement across subterranean salt and anhydrite sections which would be partially dissolved if fresh water cement compositions were used. Also, in cementing formations which include highly sensitive swellable clays saturated salt water cement compositions are preferred in that such clays are less sensitive to saturated salt water than to fresh water. Saturated salt water cement compositions are generally also more compatible with oil based muds than fresh water compositions.
Thus, there is a need for improved set retarded cement compositions and methods of using such compositions where the compositions can be formed using any normally available water including saturated salt water.